It is recalled that distance measuring equipment of the DME type is usually used as an aid to aerial navigation, both en route and during approaches. The function of such equipment is to provide, on interrogation, the distance which separates an aircraft from a ground station (also called a transponder or radionavigation beacon) whose position is known.
Such equipment operates as follows: the aircraft carries an interrogator which interrogates the ground station. The interrogation message consists of a pair of VHF pulses whose spacing and carrier frequency are defined by the ICAO (International Civil Aviation Organization), depending on the type of DME and its location which are known to the transponder. When the transponder receives and recognizes these pulses, it emits a response destined for the aircraft. The response also takes the form of a pair of pulses of defined spacing and carrier frequency, emitted with a likewise defined delay, the whole being fixed by the standards of the ICAO and therefore known to the interrogator. When the interrogator of the aircraft receives and recognizes this response it deduces the distance which separates it from the transponder from the duration of the outward-return journey of the pulses.
The terrestrial surface of the globe is meshed by a more or less dense network of beacons. The position of these beacons is known and stored in a database onboard the aircraft. At each instant, only a small number of these beacons is accessible to the aircraft to provide it with a distance measurement, one speaks of eligible beacons.
It is known that the measurement of the altitude of the aircraft by distance measuring equipment of the DME type is inaccurate because of the ground position of the beacons, this is the reason why the aircraft altitude measurement is carried out, in general, by some other means as for example, an anemo-barometric probe. In this case, the locating of the aircraft by the distance measuring equipment of the DME type amounts, when the measurements are carried out with an infinitely large accuracy, to a two-dimensional problem that can be solved by virtue of measurements of distance separating the aircraft from two beacons.
Represented in FIG. 1 is the principle of locating the aircraft on the terrestrial surface, by making the assumption of a two-dimensional world: a measurement of the distance separating the aircraft from a first beacon (BX) projected onto the terrestrial surface equals dlmX, and a measurement of the distance separating the aircraft from a second beacon (BY) projected onto the ground equals dlmY. The intersection of the circle of radius dlmX centered on the position of the beacon BX and of the circle of radius dlmY centered on the position of the beacon BY provides an estimation of the 2D terrestrial position of the aircraft PTEA.
Hereinafter, the 2D terrestrial position of an object or point is defined as the location of the object or point in a terrestrial reference frame, which is not necessarily plane, when its altitude is considered to be zero. The 2D terrestrial position can for example be expressed in the form of a longitude value and a latitude value.
In reality, the accuracy of a distance measurement delivered by a beacon is not infinite. It is possible to show that, in the case where N distance measurements of identical accuracy (with N greater than or equal to two) are carried out simultaneously employing N beacons, the accuracy of the estimation of the 2D terrestrial position of the aircraft increases with the number of beacons employed (N), when the beacons are positioned in an optimal manner. The optimal positions of the beacons correspond to arrangements where the angles between the geodesics relating the 2D terrestrial position of the aircraft and the 2D terrestrial positions of the N beacons used are close to π/N radians. But, the duration required in order to choose an optimal configuration comprising a number (greater than or equal to two and not fixed a priori) of beacons from among a number of eligible beacons which may exceed about forty is prohibitive. One prefers therefore to limit oneself to searching for a pair of beacons, arranged in an optimal way, from a list of eligible beacons.
In the prior art, the estimation of the 2D terrestrial position of the aircraft at an instant t2 implements a method of selecting a pair of beacons which searches for, on the basis of the knowledge of the 2D terrestrial position of the aircraft at an instant t1 prior to t2 and of the position information for the beacons, contained in the database, the pair of beacons whose measurements of the distances which separate them from the aircraft are capable of producing the most accurate estimation of the 2D terrestrial position of the aircraft at this instant. The beacons making up the pair are those which have a 2D terrestrial position such that the angle (Δθ) formed by the geodesics connecting the 2D terrestrial position of the aircraft to the 2D terrestrial positions of each of the beacons used is closest to π/2 radians.
This method has the advantage of providing, at any instant, a measurement of the 2D terrestrial position of the aircraft which is the most accurate achievable with two beacons. However, the selection criterion that the method uses exhibits the drawback, when the selection method is implemented in a repeated manner, of producing a frequent change of one or more selected beacons, for example in the case of the aircraft overflying a terrestrial zone dense with beacons. Now, a beacon modification requires a duration of initialization, that may be up to five seconds, which is related to a change of carrier frequency of the message emitted by the interrogator and this duration of initialization reduces the availability of the estimation of the 2D terrestrial position of the aircraft. Additionally, modifying the pair of selected beacons is detrimental to the continuity of the position estimation of the aircraft over time since it disturbs the setting up of processing operations allowing estimation of the biases of the beacons. A prior art solution consists in reducing the frequency of implementing the selections of the pairs of beacons by triggering the beacon selections on the basis of a criterion for modifying the current pair of selected beacons. The modification criterion can be, for example, fixing a floor value of the accuracy of the position estimation. This accuracy can, itself, be estimated by means of evaluating the angle Δθ. A beacon selection is retained so long as the evaluation of the accuracy of the estimation of the aircraft position carried out by means of the pair of selected beacons indicates that it has a value greater than the floor value. As soon as this is no longer the case a new selection of beacons takes place. But such a reduction in the beacon selection frequency based solely on the accuracy of the position estimation can make it necessary to fix a relatively low accuracy floor value and does not guarantee that the estimation accuracy value will be maintained over time. In particular, even if a selection of beacons is stable over a time period, there is no certainty that the pair of beacons selected at the start of the period is that which provides a position estimation with the best accuracy over the whole period.